This invention relates to crystal oscillators and, more particularly, to crystal controlled transistor oscillators wherein the crystal is coupled between the base of the transistor and a point of reference potential and a voltage divider capacitor is coupled in parallel with said crystal between the base and reference potential with the junction point of the capacitor divider coupled to the emitter of the transistor. Oscillators of this type have been known for a number of years, for example, see Angel, U.S. Pat. No. 3,256,496 and Mrozek, U.S. Pat. No. 3,641,461.
In the prior art oscillators, the ratio of the above-described voltage dividing capacitance is such that the capacitance between the base and the emitter is equal to or usually larger than the capacitance between the emitter and the reference potential.
In the prior art, this capacitance between the base and the emitter is made large in parallel with the input transistor so as to control by swamping the base-to-emitter diffusion capacitance. This diffusion capacitance changes with temperature and should be minimized so that the oscillator frequency is not so dependent on temperature. Crystal frequency change or trimming in mobile applications is necessary to compensate for crystal aging and offset production tolerances. With present art oscillators, this trimming is accomplished by varying the negative reactance as seen by the crystal (load capacitance). Present art oscillators trim this frequency by varying the capacitance in series with the crystal or by varying the emitter to ground capacitor while maintaining a large capacitance shunting the transistor input. The merit of having the capacitance in series with the crystal is that there is a large PPM/PF (parts per million per picofarad) frequency deviations but this arrangement is most sensitive to frequency trimming effects. See above cited Mrozek U.S. Pat. No. 3,641,461. Since the capacitance in series with the crystal is most sensitive to frequency trimming effects the alternative of the varying part of the emitter to ground capacity is used. This technique however creates additional noise sources by introducing the compensation thermistor resistor in series with the crystal which degrade the circuit Q or dictates a certain relatively high value of emitter resistance to minimize the frequency trimming effects. Another reason why the minimization of the noise sources is difficult is because large values of emitter resistance are necessary to limit the frequency range dependency of the magnitude of the relative excursion of the reactive part of the input impedance of the oscillator. This characteristic is very important in mobile oscillator applications wherein the same basic circuit has to cover a frequency octave in most cases (usually 10 - 20 MHz at the fundamental of the crystal). The lack of uniformity of this reactance vs. frequency would require a range of component value with frequency of oscillation making production less practical and/or more expensive. A large capacitance between base and emitter of the oscillator transistor for a given load capacitance has the effect to reduce the amount of voltage available across the crystal and therefore a relatively lower signal is obtained at the output. Since the above noise sources have a limit of minimization, any reduction of the power output also limits the maximum signal-to-noise ratio obtainable directly from the crystal oscillator.
Although in the prior art, there is an oscillator of the type with the ratio of the voltage dividing capacitors across the crystal being such that the capacitor added across the base-to-emitter junction is less than that across the emitter ground, (see, for example, U.S. Pat. No. 3,528,032 of Tahmisian, Jr. et al.) this value of capacitance appears to have been selected to emphasize the harmonic content. Moreover, the particular transistor device involved has a relatively large base-to-emitter diffusion capacitance and hence the total value of the impedance across the base-to-emitter electrodes becomes relatively small with respect to the value of impedance between emitter and the reference potential. This effectively reduces the portion of the signal existing across the crystal that is transferred at the transistor input results in lower signal level at the output of the oscillator device and consequently produces a relatively low signal-to-noise ratio.